The present invention relates to a process in the integrated circuit fabrication art, and particularly concerns the formation of contact regions and metallurgic barriers thereover.
In the fabrication of integrated circuits, silicon is commonly used in its monocrystalline form as a substrate and in its polycrystalline form for gate electrodes and interconnects. Aluminum is commonly used as a conductor and especially for forming contacts or interconnects. One of the problems is that silicon is soluble in or diffuses into some metals such as aluminum. Thus, when a contact is formed by opening a contact window to, for example, a source or drain region formed in a silicon substrate and aluminum is used to fill the contact opening, some of the silicon dissolves, resulting in what is generally called a "spike" which extends downward from the contact. If the spike goes all the way through the source/drain region or other underlying region, then the transistor will be ruined by a "spiked junction."
To prevent this, titanium nitride can be used as a metallurgic barrier against the reaction between the silicon substrate and aluminum contact material. Thus, when the aluminum is separated from the silicon by a titanium nitride layer, spiking is reduced or stopped entirely. One can form titanium nitride by reacting pure titanium with nitrogen, but it is difficult nevertheless to obtain a good barrier quality. First, one desires a barrier layer which is not too thin. Second, the barrier layer formed of titanium nitride should have trace amounts of impurities. It is believed that impurities such as oxygen incorporated in the barrier improve the barrier quality by inhibiting diffusion of silicon atoms through the barrier along titanium nitride grain boundaries. Also, the barrier layer should not be in a state of tensile stress, as a stressed layer is believed to be less effective in preventing diffusion of silicon. Lower tensile or compressive stress is preferred.
Ordinarily, after titanium is deposited over a contact opening to a region in a silicon substrate, when titanium nitride is being formed, by a reaction in a nitrogen atmosphere, some of the titanium in the contact opening will react with the silicon to form titanium silicide. The titanium silicide forms a low resistance contact to the region in the substrate. Titanium silicide, however, is not a barrier material and can cause the juction to leak if the amount of titanium silicide is excessive.
Moreover, titanium silicide forms relatively quickly compared to titanium nitride, and as a result, if titanium is placed over a contact opening and reacted in a nitrogen atmosphere, a relatively substantial amount of titanium silicide forms compared to the amount of titanium nitride. The rates of the titanium-silicon reaction and the titanium-nitrogen reaction determine the thickness of these two layers. Because these two rates cannot be controlled independently, it has not been possible to control the relative thickness of these two layers when they are simultaneously formed. In fact, one desires to obtain only a thin layer of titanium silicide because any more would create high tensile stress in the contact.
Another problem with the prior art is that it does not introduce controlled amounts of oxygen or other elements into the titanium nitride film when titanium nitride is being formed by direct thermal reaction. The oxygen or other elements are believed to fill the grain boundaries in the titanium nitride.
It therefore is the object of the present invention to overcome these problems of the prior art processes.